It is known that reduction of manufacture costs and applicability to a wide market of the modern mobile telecommunication network has been rendered possible thanks to the microelectronic circuit manufacture.
The typical example is the mobile or cellular telephone, where the whole communication system is substantially made in a single chip. The microelectronic manufacture has imposed a series of design constraints and it has created manufacturing problems which, in turn, have had the benefit to spur the scientific and technological search for the solution thereof.
Presently, the modern mobile systems use a variety of transmission interfaces, including GSM (Global System for Mobile Access), TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access). In these systems a low noise local oscillator is required. A low noise local oscillator in optical receivers is similarly necessary.
Voltage controlled oscillators or VCOs represent a decisive element for making synthesised signal generators through their use in Phase Locked Loops or PLLs. In these devices a reference generator, generally a un quartz crystal oscillator with oscillation frequency considerably lower than of the VCO, controls the precision of it through the comparison of the two relative phases and through the generation of an error signal that modifies the VCO frequency, correcting possible deviations.
U.S. Pat. No. 4,063,193 discloses a differential oscillator comprising a parallel resonant circuit, operating as oscillator time reference, that is separated form both the ground and the power supply, and it is power supplied in a differential way by two transistors, controlled by the same voltage across the resonant circuit, so as to operate as switches, supplying power in alternate way to the two ends of the resonant circuit with a current generally produced by a current generator.
Many implementations of this circuit exist, in particular with active loads, and implementations with both MOS and bipolar transistors have been presented.
The differential one is certainly the configuration more used in integrated circuits. In this regard, the differential architecture represents the basis for making VCO oscillators, since the differential connection of the oscillating circuit easily allow the addition, in parallel to the resonant circuit, of the series of two equal variable, voltage controlled, capacitors: for the existing symmetry, the central node between the two capacitors is devoid of differential signal and it may hence be connected to a control voltage, capable to modify the capacitance of a diode or MOS, thus allowing the oscillator frequency to be controlled.
U.S. Pat. No. 5,955,929 discloses a differential VCO oscillator resistant to the power supply voltage noise, comprising a VCO, based on a delay signal, including a cascade of delay cells, each one comprising a compensation circuit and a reference current source. The compensation circuit adjusts the capacitance of the delay cell for compensating the noise introduced by the power supply.
U.S. Pat. No. 6,140,880 discloses a circuit architecture comprising a VCO and a circuit that, particularly during power up and initial acquisition of the phase to be locked, prevents the oscillator from oscillating outside a predetermined frequency range. In particular, the circuit is particularly complex and comprises a frequency and phase detector, driven by a oscillating reference signal, that generates through a proper filter a control voltage of the VCO, from which it draws a feedback signal, through a frequency divider.
U.S. Pat. No. 6,542,043 discloses a voltage controlled differential oscillator comprising a resonant LC circuit, including a pair of variable capacitance components and an inductor, provided with a central node connected to the circuit ground through a resistor. In particular, the oscillator amplification and bias are adjusted through a corresponding adjustment of the resistance connecting the inductor central node to ground.
U.S. Pat. No. 6,653,908 discloses a differential oscillator generating an oscillating signal the amplitude of which is automatically controlled for selectively minimising the phase noise. In particular, the differential oscillator, including two symmetrical LC resonant branches, comprises a current source that is controlled by a voltage signal resulting from the processing of a negative peak detector and of a positive peak detector, in order to adjust the amplitude of the oscillating signal generated by the oscillator.
US Patent Application No. US2004085144 discloses a VCO oscillator made as differential oscillator comprising a plurality of transistors operating as switches, and a parallel LC resonant circuit, a current source for supplying the oscillating circuit, and a biasing circuit coupled to both the LC resonant circuit and the current source. In particular, the LC resonant circuit comprises a pair of variable capacitance diodes or varactor parallel connected to an inductor, provided with a central node power supplied by a bias dc voltage.
In spite of the control and correction mechanism with which a Vco is provided, the synthesised source is not ideal: random frequency variations around the nominal value are present. These variations constitute an enlargement of the source spectrum. The portion of spectrum ranging from tens of KHz up to some MHz from the carrier is generally the one of greatest interest for telecommunication. The presence of phase noise in this band entails a mixing among close channels in many wireless applications (such as for instance GSM and WLAN).
In this interval the correction of the PLL circuit is limited, sometimes greatly, by the band of the feedback loop, that in turn is limited for stability problems. The VCO noise is hence determining in the final characteristics of the system.
Unwanted variations of frequency (and phase) of the VCO may have various origins. A first source of alteration of the oscillator evolution is due to the noise present in the electronic circuits, in particular in the parasitic resistances of the reactive components of the resonant circuit representing the oscillator time reference, partly in the circuits which must periodically supply the resonant circuit with the energy lost during the oscillation just because the parasitic resistances.
To this phenomenon, usually defined as phase noise, the possibility of deviations, due to the interaction with the rest of the circuits within the integrated circuit wherein the oscillator is contained, adds. The phenomenon of frequency variation, called VCO pull-in, is of particular relevance, which is related to the effects on the oscillator by sudden variations of the power supply voltage due to, e.g., sudden power up or power down of adjacent circuit portions with high current loads, in particular of output circuits of the transmitter part, normally integrated within the same circuit for the reasons of savings.
Consequently, the possibility to design VCO with low phase noise is of particular interest in developing integrated circuits for telecommunication.
Document EP 0 899 866 A discloses an oscillator circuit including differential amplifier, having resonant means supplied by a current source, to which MOSFET varactors are coupled, wherein the capacitance of the MOSFET varactors, and therefore the oscillating frequency of the oscillator, is controlled by a tuning voltage applied to a common node of the MOSFET varactors.
Document EP 1 093 215 discloses an oscillator circuit having resonant means and an active stage electrically connected to and injecting power to the same at instants when the maximum value of the output oscillating signal occurs.
However, no one of the oscillators available nowadays, among the previously mentioned ones, has a low phase noise, though some of them have rather complex circuit architectures.